I have a JM stamped 336 in 30-30 that I bought used last year, it is probably an 80's or 90's gun just going off appearance but I've never tried to verify that.

Sometimes when I've fired a round it is VERY difficult to get the action moving, like it's stuck shut. Once it breaks loose and starts moving everything works smoothly like it should. This doesn't happen with every round, sometimes it works great, but the last time I had it out I tried 4 rounds and after all of them it "stuck" shut and required a lot of effort. All with factory ammo, although me selection is very limited as I got it right before the ammo panic kicked in.

I initially thought maybe the chamber had some rust forming or buildup and needed a good cleaning so I worked it over with a bronze brush, some oil and swabbed it out so it's definitely clean now, but that didn't fix it.

I had it all apart to clean and lube when I first got it and did not notice anything that looked worn or had burrs, nothing that seemed suspect.

I'm open to suggestions or ideas on what I might need to look at!

Easiest thing first: make sure the screw behind the loading gate is tight. That's the exact problem a loose one can cause.

If that isn't the problem, then the next time it sticks closed use a small screwdriver to move the extractor off the rim of the case. Then open the bolt with the lever, and then knock the case out with a cleaning rod. That'll tell you if the problem is cases sticking in the chamber. Then you'll have to figure out why the cases are sticking.

Thanks for the tip on the screw, haven't had time to check it yet but I sure wouldn't have thought of that causing this issue, and the other idea is a good one too!

I would blast the works with brake cleaner and hit it with high pressure air.  Then lube and try again.

Sometimes a bit of debris or maybe brass from a cleaning brush can hide in places and jam things up.

I tend to do this with complicated guns that I really don't want to take apart.

Little update, screws are all tight which is what I expected, so I'll have to go do some test firing when I get time.

The gun should be clean, it did this both before and right after my initial takedown and thorough clean and lube after purchasing it. The only unusual thing about this gun I didn't mention is that I am shooting it suppressed, so backpressure will be higher although I don't expect that to be the issue.

For year of manufacture take first 2 digits of serial # and subtract from 100.
ie 25xxxx is 100-25=75 so 1975

I had the same problem before and it was the screw, as mentioned above. Literally a 2 second fix.

OP, what was the end-game?

I had the same problem. It was because of a replacement bolt.  The headspace was off. It worked fine except for hard extraction. Check to see if the bolt matches, it should have a serial number, and also check the spent brass if you see any swelling down by the head stamp

I still haven't taken time to tear it down again or go shoot more. Been too busy with farming and side businesses.

I appreciate all the input from everyone, hopefully I'll look into it soon and report back.

Ok finally went and shot the Marlin today and confirmed that it is cases sticking in the chamber and not the action. I guess I'll try scrubbing the chamber again, maybe see if I can find a way to get some fine steel wool on the drill in there, not sure yet.

Well I went at the chamber with 0000 steel wool wrapped around a bronze brush chucked up in the drill and figured I had it made now. Next time I went to my range the first round stuck.

I have been shooting PPU ammo, but I'm thinking next time I'm gonna try some of the Federal Fusion rounds I have. Those 2 are all I have on hand though, and not much of either.

Check case length. Some brands or lots tend to be on the long side, which can cause sticking when the case mouth expands into the throat / rifling.
While it's a different rifle and chamber, SMG put out a cautionary note re. their FG42 and PPU ammo for this very issue.

Good suggestion, I was just going over a fired case vs new round with my calipers looking for any swelling or stretching.

1 each of unfired and the fired PPU (used the exact case that was stuck) both measure 2.033", unfired Federal Fusion also measures 2.033", I don't recall the Fusion ever sticking on me at all, but I only shot it for sight in and 1 round on my first deer last year so not a thorough test by any means.

It may be loaded a little Hot what do the primers look like compared to the Federal?

I haven't noticed any signs of flattening or pushed out primers just looking the PPU cases over, but I have not compared directly to the Federal, I will have to do that later!

Today I took it out and ran 10 rounds of the Federal Fusion through it, maybe a couple of them seemed a little stiffer to extract than normal but they all extracted ok. Then I tried some 170gr PPU, (I had been shooting 150gr PPU) and the 2nd or 3rd round stuck bad enough I couldn't open the action so I was done for the day. So it would seem it is an issue with the PPU ammo, which is a bummer because that's most of what I have.